Ropes are used in a variety of applications where it is desired to know the loading history, either the highest load or a complete record of each significant load to which the rope has been subjected. Marine mooring and towing are examples of such applications. The marine environment is especially severe, and conventional methods of measuring and recording rope loads are not generally satisfactory.
Electronic load measuring and recording devices incorporating stain gages or other electronic differential motion sensing elements, such as those disclosed by R. D. Karl in U.S. Pat. No. 3,956,742 and S. S. Fishfader in U.S. Pat. No. 4,283,942, have been proposed to record tensions in ropes in the marine environment. These utilize electronic strain gages which require supporting electronic signal driving, processing, and recording devices, as well as batteries to power this electronic equipment. Such electronic devices are costly to manufacture and difficult to install, operate, and maintain. Thus they are not generally suitable for prolonged service in a rough marine environment.
Mechanical scratch recording strain gages, such as that disclosed by R. H. Prewitt in U.S. Pat. No. 3,430,250, are rugged, can be made water resistant, and do not require electric power. These mechanical scratch strain gages are not as sensitive to strain and must be mounted on surfaces which undergo significant strain in order to obtain adequate reliable records.
Strain gage might be mounted directly on the rope thimble body. The surfaces of a thimble body which are highly strained and thus might be suitable for application of strain gages are also subjected to heavy pressure and abrasion, both by the rope and by connecting devices, which would destroy any strain gages mounted on these highly strained surfaces.
Other surfaces of the thimble body are not highly strained and are thus not suitable for strain measurement and recording. Mechanical strain recorders especially require larger relative differential motions than could be generated in those surfaces of a conventional thimble body which are remote from potential damage by the rope or the connection element.
This invention is a simple, inexpensive way of incorporating a differential motion sensing and load recording device into a rope thimble, such that it experiences sufficient differential motions to adequately respond to and record loads applied to the tension member, and yet it is positioned away from the rope and from connecting devices placed in the hole which might cause damage.